Controlling Microbes

Not Too Hot to Handle

11

Physical Methods of Control

• Fire is a great sterilizing agent• Heat alone, though cannot inactivate spores• Radiation is a great sterilizing agent• Deinococcus radiodurans resist high levels of

radiation, too, though!

Physical Methods of Control

• Heat methods– Denature and inactivate

proteins

– Drive off necessary water– 100 °C steam from boiling

water (moist heat)• Cannot inactivate

spores– Pressure

• Autoclave• 15 psi• Allows higher water

and steam temperatures

• 121 °C steam now capable of inactivating spores

Figure 11.2: Operation of an autoclave

Physical Methods of Control

• Heat methods– Pasteurization

• 62.9 °C for 30 minutes (hold method)

• 71.6 °C for 15 to 30 seconds (flash method)

• 82 °C for 3 seconds (ultraflash method)

• Used to kill pathogens in milk, wine, fruit juice

• Does not inactivate spores

• Protects against Mycobacterium tuberculosis, Coxiella burnetii

– Dry heat• 160 to 170 °C for at least 2 hours

• Oxidation of proteins

• Necessary for materials that cannot be autoclaved or pasteurized

Physical Methods of Control: Heat

Figure 11.3: Temperature and the

physical control of microbes

Physical Methods of Control

• Radiation– Ultraviolet radiation

• Results in mutations

• Effective against spores, since no repair mechanism

– Ionizing radiation• X rays

• Gamma rays

• About 10,000 times more energetic than UV light

• Sterilizing

• Electron beams

– Room temperature treatment

– Can pass through packaging to sterilize contents

Physical Methods of Control

• Drying– Also known as desiccation– Water required for microbes to survive– Removal prevents many enzymatic processes– Not effective to inactivate spores– Effective for storage of

• Cereals

• Grains

• Other foodstuffs normally stored in pantries

Physical Methods of Control

• Filtration and refrigeration– Filtration

• Heat-sensitive solution passed through filter

• Pores in filter prevent passage of microbes

– Pores can be chosen based on size of microbe

– 0.2 m to 0.5 m pores prevent passage of many bacteria

– Does not prevent passage of viruses

• Solution is not truly sterilized

– Refrigeration• Slows down enzymatic reactions

• Only slows microbial growth

• Refrigerated foods are not sterile

Chemical Methods of Control

• Disinfection and antisepsis• Practiced for thousands of years• Medicinal chemistry started in the 1800s• 1860s: Joseph Lister

– Principles of antisepsis in surgery– Diminished incidence of common infections that occurred during

surgery

Figure 11.6: Joseph Lister

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Chemical Methods of Control

• General principles– Disinfectants

• Kill microbes on inanimate objects

– Antiseptics• Kill microbes on body surfaces

– Ideal agent• Soluble in water• Kills all microbes• Stable over time• Nontoxic to humans and animals• Uniform composition• Combine with organic matter other than microbes• Highest efficacy at room or body temperature• Efficiently penetrate surfaces• Not corrode or rust metals• Not damage or stain fabrics• Readily available in useful quantities• Reasonably priced

Chemical Methods of Control

• Alcohols and aldehydes– Alcohols

• 70% ethyl alcohol (ethanol)

• Isopropyl alcohol (isopropanol)

– Aldehydes• Formaldehyde (formalin)

• Glutaraldehyde

Chemical Methods of Control

• Detergents and phenols– Detergents

• Strong wetting agents

• Surface tension reducers

• Dissolve microbial cell membranes

– Phenols• Also known as phenolics

• Lysol

Antibiotics

Figure 11.14: The sites of activity of various antibiotics on a bacterial cell

Antibiotics

• The first antibacterials– Paul Ehrlich

• Magic bullets

• Harm bacterial pathogens and not host

• Arsphenamine

– Firs syphilis treatment

– Contains arsenic

– Gerhard Domagk• Prontosil

– Active ingredient: sulfonalamide

Antibiotics: Sulfonilamide

Figure 11.10a,b,c: How sulfanilamide works to kill bacteria

Antibiotics

• Cephalosporins and aminoglycosides– Cephalosporins

• Like penicillin

• Produced by Cephalosporium

• 6-membered ring, as opposed to penicillins’ 5-membered ring

• Cephalexin ( trade name Keflex)

• Cephalothin (Keflin)

• Cefotaxime (Claforan)

• Ceftriaxone (Rocephin)

• Ceftaxidime (Fortaz)

Antibiotics

• Cephalosporins and aminoglycosides– Aminoglycosides

• Useful against Gram-negative bacteria

• Streptomycin

– Major early weapon against tuberculosis

– Now most Mycobacterium tuberculosis is resistant

• Most produced by Streptomyces

• Inhibit protein synthesis

• Gentamicin

• Neomycin

Antibiotics

• Broad-spectrum antibiotics– Inhibit or kill many different microbes– First one discovered: chloramphenicol

• Extremely toxic

• Still used in dire situations

– Tetracyclines• Minocycline

• Doxycycline

• Used especially for Gram-negative infections

• Few side effects

– Resistance

– Fungal superinfection

– Light sensitivity

– Deposition in teeth

Antibiotics

• Other antibiotics– Macrolides

• Inhibit protein synthesis

• Erythromycin

• Azithromycin (Zithromax)

• Clarithromycin (Biaxin)

– Vancomycin• Inhibits cell wall synthesis in Gram-positive bacteria

• Severe side effects

– Streptogramins• Quinupristin plus dalfopristin (Synercid)

Antibiotics

• Other antibiotics– Rifampin

• Inhibits RNA polymerase• Synthetic• First used against M. tuberculosis• Useful against Neisseria, Haemophilus

– Bacillus-produced antibiotics• Only used topically because of toxicity• Bacitracin

– Inhibits cell wall synthsis– Effective against Gram-positive bacteria

• Polymyxin B– Inhibits plasma membranes– Effective against Gram-negative bacteria

Antibiotics

• Antiviral and antifungal antibiotics– Antiviral chemicals

• NOT antibiotics• Amantadine• Acyclovir

– Antifungal antibiotics• Nystatin

– Useful against Candida albicans– Reacts with sterols specifically present in fungal membranes

• Griseofulvin– Ringworm

• Amphotericin B (Fungizone)– Fungal infections of internal organs

• Imidazoles– Clotrimazole (Lotrimin)– Miconazole (Monistat)

Antibiotics

• Antibiotic resistance– Spreading through bacterial populations

• Bacterial pneumonia• Streptococcal blood disease• Gonorrhea• Staphylococcal infections• Tuberculosis

– Means of resistance• Destruction of antibiotic• Prevention of uptake• Alteration of metabolic pathway• Mutation that prevents antibiotic binding or efficacy

Antibiotics

• Antibiotic resistance– Overuse of antibiotics– Overdose of antibiotics– Abuse in developing countries– Use in animal feeds– Resistance gene transfers from one bacterium to another

• Shigella• Salmonella• Staphylococcus

– Alternatives to reduce resistance or increase efficacy• New antibiotics• Limited antibiotic use• Phage therapy